Detach prepared execution ownership from runners

internal/usecase/runner.go

@@ -43,6 +43,7 @@ var (
 
 // Runner executes the Promptkit core use case.
 type Runner struct {
+	identity                *runnerIdentity
 	promptDefs              promptdef.Repository
 	profiles                profile.Repository
 	artifacts               artifact.Reader
@@ -54,6 +55,10 @@ type Runner struct {
 	defaultTransientRetries uint64
 }
 
+// A non-zero-sized token gives each constructed runner a distinct identity
+// without retaining its collaborators in prepared handles.
+type runnerIdentity struct{ marker byte }
+
 // RunAdmitter reserves capacity for one resolved backend run.
 type RunAdmitter interface {
 	Admit(context.Context, string) (func(), error)
@@ -113,6 +118,7 @@ func NewRunnerWithRepairer(
 		retries = defaultTransientRetries[0]
 	}
 	return &Runner{
+		identity:                &runnerIdentity{},
 		promptDefs:              promptDefs,
 		profiles:                profiles,
 		artifacts:               artifacts,